It's Not Easy Being Green

Three-year-old Nathan Noyens dumped his little wooden cart of alphabet blocks on the floor, barely missing Nora Shekrie's second and third toes, vulnerable in the one pair of spring sandals in her wardrobe that wasn't in the bag waiting to go to Goodwill. His mother, Anna Noyens, continued to regale Nora with evidence of Nathan's overall superiority to other children his age -- or any age, for that matter.
"Nene knows his letters already, and he's off the top of the charts in his motor skills. Dr. Pfeffer says he's the smartest baby he's seen in years. Last week, he figured out how to open the child-proof latches on the cereal cupboard. He ate half a box of Cheerios before I noticed."
Desperate for an escape, Nora took another sip of her tea as she looked around at the decor, a pleasant, understated mixture of soft greens and browns. She had no doubt that Nathan would spend long periods tracing the ivy and brick lines stenciled on the cream-colored walls; the pattern brought back memories of wallpaper from Nora's own childhood.
"Anna, I love what you've done with this room. I remember how it looked when the Stetsons lived here. The family was warm, but the room couldn't stand on its own. Now, it looks like something out of a magazine. How did you *find* these colors?"

"Natural talent, I'm afraid." Anna enjoyed Nora's puzzlement for a triple heartbeat. "I'm color blind: classic red-green. I've learned to ask people what the colors are, and I've studied color coordination in books until my eyes couldn't tell white from black." She sounded wistful, almost resigned to living with this over-educated aspect of herself. "I think I know a hundred ways to ask about a shade of green, and to judge how other people see it."
Only someone with less sensitivity than the painted brick wall would have missed the combination of pride and pain in her voice. Nora patted her arm, looking worriedly at Nathan.
Anna's voice brightened. "Oh, don't worry about him, Nora. Nathan is fine. He can tell his colors just fine." Nathan was just placing a green "N" on a red "T" block in the cart.
"Nathan, dear," she cooed, "bring Mommy the green block, please?"
He looked at his mother, then at the blocks around him, finally focusing on the top of his two-letter stack. He clutched the "N" and brought it over to her.
"See, Nora? Just fine. Richard's color vision is incredible, like his mother's. In fact, he did most of the detailed color work in this room. I learned all the relationships, and he got the colors just right, from what everyone else says. Anyway, we've been teaching Nathan his colors for a couple of weeks now, and he's getting them better. He's ahead of schedule, like I said, but he really knows his red from his green."
Nora looked at the set of blocks, the red truck, the green frog, ... the entire variety of bright colors certainly gave an air of certainty to Anna's claim.
"So, Anna, you haven't had him tested professionally?"
"No, it hasn't been necessary. We worried, of course. It's a 1-in-4 chance, but he's just fine."
"I'm afraid not, Anna. He certainly seems smart, but I don't think he can tell red from green any more than you."

What did Nora conclude that had escaped the doting mother?

Hint

Answer

Color blindness is both recessive and sex-linked to the X chromosome. A color-blind woman carries the trait in both chromosomes of her 23rd pair. Since a son gets his only X chromosome from his mother (a Y from his father), all her sons will inherit her color blindness.
Apparently, Nathan had learned that the first letter of his name was green. Similarly, he had learned that his truck was red, his frog was green, and so on.Hide

There is one error in this teaser..Ther is a slim possibility that Nathan is not colour blind. It is true that colour blindness is X-linked, but in a woman, only one of the X-chromosones is active in any one cell. They form what is called Barr bodies. This is why women have to be tested in both eyes for colour blindness, unlike men who only need one eye tested ( It is possible for a woman to be colour blind in just one eye). So a woman could carry just one X chromosone with the colour blindness gene and by pure luck, exhibit coulour blindness, yet transfer the non-colourblind gene to her son.... Otherwise a great teaser.

And JZumun, even though color-blindness is a recessive trait, the key is that it is x-linked. A male has a sex chromosome pair of XY, while a female has an XX.
So if the gene for color-blindness is X-linked, and a male only has one X, there is no other gene to express.